Good decisions require good information. For GIS users, the quality of information coming out of their systems depends on the accuracy of the data going in. Confident decision making leaves no room for doubt over data accuracy. The last thing GIS users need to worry about is whether the data collected by GPS meets the accuracy requirements of the intended application.
Given the large number of products on the market, selecting the right GPS receiver can be difficult. With budgets tightened everywhere, some GIS users have begun looking at and buying less expensive recreational GPS products that are popular with outdoor enthusiasts. New features, such as ruggedized cases and differential correction, and a price tag often below $500 make these units attractive compared with higher-priced professional-grade GPS receivers.
But beware, as is true with most products, you get what you pay for. There is a significant difference in the accuracy of location data acquired by recreational GPS receivers versus the professional units. The 10-meter error typical of a recreational model won't cause a major problem for a hiker in the woods, but such inaccuracy may not be acceptable for GIS applications.
Recreational and professional GPS units are designed and built for different purposes. A recreational GPS unit is designed to acquire a location fix quickly without the need for pinpoint accuracy because hikers can find their campsite once they get within 10 meters of it. GIS users, on the other hand, typically require extremely accurate placement of features often to within a meter or less so that data layers can be overlaid and intricate spatial relationships can be determined.
Although recreational products are not specifically designed for GIS mapping, they can be used successfully in some applications. And for some GIS users, the recreational products may be the most cost-effective choice. In choosing between a recreational and a professional GPS receiver, GIS users should answer the following questions to be certain the selected unit will meet their application needs.
* Do you need to integrate data seamlessly with a GIS?
If you will be converting GPS points to a specific GIS format, such as shapefile format, you should purchase a professional-grade GPS receiver. Some newer units can even convert points to popular GIS formats on the fly during downloading. Most recreational receivers cannot convert data to other formats.
* Will you be collecting attributes along with location points?
Many GIS users have found that accurate attribute collection is just as crucial as location acquisition. Only the professional GPS products offer customizable interfaces and routines for detailed attribute collection.
* Is five-meter accuracy sufficient for your application?
A recreational GPS is typically able to achieve 10-meter accuracy in autonomous mode, but some now can handle real-time differential correction capable of sharpening accuracy to five meters or better. In this situation, the most cost-effective purchase may be the recreational unit.
* Is submeter accuracy required for your application?
For many GIS users, accuracy is measured in centimeters. In these cases, professional GPS units are the only ones capable of performing the differential postprocessing required to achieve this level of accuracy.
After price, data quality and accuracy are the main differentiators between recreational and professional units that influence the buying decision of a GIS user. Engineering, design, and construction characteristics account for the variation in capabilities among GPS receivers. Professional units have been engineered and built to acquire more accurate location coordinates. Although many design features contribute to this higher level of performance, three factors—quality control, electromagnetic shielding, and antenna technology—set GIS-grade products apart from recreational receivers.
Quality Control—Professional GPS units give users control over the quality of the position points that are collected. Through a simple interface, the user can establish specific thresholds for acceptable data quality. For instance, the user chooses the number of satellites and position above the horizon needed to achieve suitable accuracy. The user can also program the receiver to disregard any satellite signals that suffer from too much noise interference. These quality control settings essentially allow the user to filter out any potentially poor data that may degrade the overall quality of the location coordinates, resulting in greater accuracy in the final dataset.
Electromagnetic Shielding—Signals from GPS satellites are very weak and can easily be degraded by interference from nearby electronic devices such as laptop computers or personal digital assistants (PDAs). Given the fact that many GPS receivers and GPS cards are linked to computers and PDAs, this can pose a serious problem. High-end GPS products have built-in shielding technology that minimizes the effects of stray electromagnetic signals from other equipment.
Antenna Technology—Weak GPS signals require a sensitive antenna, especially when receiving transmissions in urban canyons and under tree canopies. The antennas provided with professional grade GPS units are designed to pick up signals in almost any environment. More important, high-end antennas protect against interference from multipath signals. These signals from GPS satellites have been degraded by bouncing off buildings and other overhead features on their way to the receiver on the ground. Multipath signals can significantly reduce the accuracy of location calculations. However, antennas on professional receivers recognize and filter out multipath signals.
For GIS users, settling for a receiver that collects data less accurate than is required by the GIS application will cast doubts over management decisions based on the information coming out of the system. While shopping for a GPS receiver, GIS users should honestly compare the needs of their GIS application with the GPS receivers in their price range.
